LIVING PHYSICAL GEOGRAPHY, 1ST EDITION

c. 2014 W.H. Freeman & Co. By Bruce Gervais

Chapter 14

Geohazards:Volcanoes and Earthquakes

Geohazard

A hazard posed to people by the physical Earth

Examples of Geohazards

Tsunami

A large ocean wave triggered by an earthquake or other natural disturbance

About Volcanoes

Active Volcanoes: Those that have erupted in the last 10,000 years and could erupt again

Dormant Volcanoes: Those that have not erupted for 10,000 years or more, but could awaken again

Extinct Volcanoes: Those that have not erupted for tens of thousands of years and can never erupt again

Three Types of Volcanoes

Stratovolcanoes (composite volcanoes)

Shield volcanoes

Cinder Cones

Stratovolcano (composite volcano):

A large, potentially explosive cone-shaped volcano composed of alternating layers of lava and pyroclast

Pyroclasts, or pyroclastic materials, encompass any fragmented solid material that is ejected from a volcano.

Shield Volcano

A broad, domed volcano formed from many layers of fluid basaltic lava

Shield volcanoes are much larger than stratovolcanoes, and can be difficult to identify as volcanoes from the ground.

Shield Volcano Structure

Cinder Cone

Small, cone-shaped volcanoes consisting of pyroclasts that settle at the angle of repose

The angle of repose is the steepest angle at which loose sediments can settle.

Cinder Cone Structure

Three Types of Volcanic Products

Lavas

Pyroclasts

Gases

Lavas

Temperature of about 1,000°C to 1,200°C (1,800°F to 2,200°F)

Silica content of 50% or less

Low viscosity and flows easily

Temperature of about 800°C to 1,000°C (1,500°F to 1,800°F)

Silica content between 50% and 70%

Medium viscosity

Mafic Intermediate Felsic

Temperature of about 650°C to 800°C (1,200°F to 1,500°F)

Silica content of 70% or more

High viscosity restricts its ability to flow

Lava Viscosities

Mafic Lava Types

When mafic lava solidifies into smooth, billowy lobes over the surface, it is called Pāhoehoe.

When it takes on a blocky, rough surface, it is called A̒ʻā.

Lava Lakes

Types of Pyroclasts

Volcanic ash: Fine volcanic powder consisting of pulverized rock particles and solidified droplets of lava

Lapilli: Marble-to golf ball-sized cooled fragments of lava

Types of Pyroclasts

Pumice: A lightweight, porous rock with at least 50% air content

Types of Pyroclasts

Volcanic bombs: A streamlined fragment of lava ejected from a volcano that cooled and hardened as it moved through the air

Volcanic blocks: A fragment of rock from the volcano’s cone that is ejected during an explosive eruption

Volcanic Gases

Gas is not a pyroclastic material, but gas emissions produce pyroclasts.

As gas forcefully exits a volcano, it blasts lava and rock debris into the air, generating pyroclasts.

Volcanic Landforms

Volcanic landforms are typically very conspicuous on Earth’s surface.

Some common volcanic landforms are volcanic mountains, columnar jointing, large igneous provinces, and calderas.

Columnar Jointing

A geometric pattern of angular columns that forms from joints in basaltic lava during cooling

Large Igneous Province

An accumulation of flood basalts that covers an extensive geographic area

Caldera

A large depression that forms when a volcano’s magma chamber empties and collapses after the volcano erupts

Caldera Formation

Calderas

Two Kinds of Eruptions:

Effusive: A nonexplosive eruption that produces mostly lava, (e.g., shield volcanoes)

Explosive: An eruption that sends rock, ash, and volcanic gases high into the troposphere, or even into the stratosphere, (e.g., many stratovolcanoes)

Ranking Volcano Strength: The Volcanic Explosivity Index

(VEI) Ranks volcanic

eruption magnitude based on the amount of material a volcano ejects during an eruption

Volcanic Intensity Comparison

Volcanoes' Two Greatest Threats

Lahars

Pyroclastic Flows

Lahar A thick slurry of

mud, ash, water, and other debris that flows rapidly down a snowcapped stratovolcano when it erupts

Pyroclastic Flow A rapidly moving avalanche of searing hot

gas and ash

The Destruction of Pompeii

Can Scientists Predict Volcanic Eruptions?

Scientists can sometimes predict an eruption within weeks or months if a volcano gives warning signs.

The Pacific Ring of Fire

More than 60% of the Pacific Ocean’s margins are subduction zones with active and dangerous stratovolcanoes.

Although shield volcanoes are found in the Pacific Ring of Fire, explosive stratovolcanoes are the most common type of volcano there.

Tectonic Hazards: Faults and Earthquakes

Although usually less noticeable than volcanic hazards, earthquakes are as dangerous as volcanoes, or even more so.

Three Types of Faults

A result of tensional force (extension) as two pieces of Earth’s crust, called fault blocks, are pulled apart

Results from compressional force, which pushes one block upward in relation to another block

Normal Fault Reverse Fault Strike-Slip Fault

Occurs where one block moves horizontally in relation to another block as a result of shearing (lateral) force

Three Types of Faults

Fault Indicators

Reverse and normal faults create a fault scarp, or cliff face, that results from the vertical movement of the fault blocks.

Where strike-slip faults cross linear features, those features may be offset by fault movement, (left-lateral and right-lateral).

How Do Earthquakes Occur?

Earthquakes occur when geologic stress exceeds friction, and the crust breaks (either along a preexisting fault or along a new fault), and the blocks move.

As each block moves, the built-up stress energy is released and travels through the crust as seismic waves, resulting in an earthquake.

Earthquake Terminology

Focus: The location of initial movement along a fault during an earthquake

Epicenter: The location on the ground’s surface immediately above the focus of an earthquake, where earthquake intensity is usually greatest

Foreshock: A small earthquake that sometimes precedes a larger earthquake

Aftershock: A small earthquake that follows the main earthquake

Geographic Patterns of Earthquakes

Most earthquakes occur along plate boundaries in seismic belts.

The tectonic settings of earthquakes include:

a. Collision zonesb. Subduction zonesc. Transform plate boundariesd. Mid-plate earthquakese. Continental riftsf. Hot spotsg. Divergent plate boundaries

Seismic Waves

Categories of seismic waves based on where waves travel:

Body waves pass through the “body” of Earth, (P Waves and S Waves).

Surface waves travel near the surface of the crust, (R Waves and L Waves).

Seismic Waves

Categories of seismic waves based on how waves move:

Compressional waves produce movement that goes back and forth in a direction parallel to the direction of the traveling waves.

Shear waves move back and forth perpendicular to the direction the waves are traveling.

Detecting Earthquakes

Earthquake magnitude is determined from measurements of ground movement using seismographs.

Seismograph (or seismometer): An instrument used to detect, measure, and record ground shaking

Earthquake Intensity

The amount of ground shaking caused by an earthquake depends on the earthquake’s magnitude, the distance from its focus, and the composition of the ground being shaken.

Earthquake intensity is determined by the amount of damage an earthquake causes to physical structures.

The Moment Magnitude Scale

An earthquake ranking system based on the amount of ground movement produced

Relies on seismographic data to quantify ground movement

Takes several weeks to collect data and calculate the moment magnitude scale because scientists have to go out and inspect the ground for indications of the extent of movement

Liquefaction Loose, wet

sediments are susceptible to liquefaction: the transformation of solid sediments into an unstable slurry by ground shaking.

Buildings resting on top of sediments may sink.

Earthquake Probabilities

Earthquake Damage

The World’s Deadliest Volcanoes

Particularly notable volcanoes in the Sunda Arc in Indonesia are Krakatau, Toba, and Tambora.

Krakatau’s most recent big eruption was in 1883, (a VEI 6 eruption that killed over 36,000 people).

Toba’s eruption about 73,000 years ago was one of the largest known volcanic eruptions in Earth’s history, (an estimated VEI 8).

Tambora

1815 eruption caused the greatest known human death toll of any volcanic eruption

Tambora’s Wide Reach

1816 was nicknamed the “year without a summer”, because it was unusually cold in both eastern North America and in northern Europe.